Modern communication systems have become extremely complex in recent years with the development of new technologies which include global satellite network links. One of the problems associated with these extremely long distance networks is the addition of an undesirable echo signal component to the information containing signal (audio, data, video, etc.).
Echo is caused when the electrical signal encounters impedance mismatches along the transmission path. As a result, a portion of the information containing signal is reflected back towards its source. This reflected signal is commonly referred to as "echo."
The most serious mismatch or defect in the transmission path, and hence the greatest source of echo, usually occurs at the receiving end. This receiving end is commonly referred to as the "customer loop." At the customer loop, the information signal is carried by a system which changes from four wires to two wires. This junction, called the hybrid interface, is the main cause of echo in the telephone transmission circuit. Therefore, efforts to eliminate echos have been directed at this interface.
Early attempts at eliminating echo employed the technique of echo suppression. A typical communications link, includes a far end speaker and a near end speaker. When the far end speaker talks, the echo suppressor switches on, and disables the return path. When the far end speaker talks, the echo suppressor switches on, and disables the return path. When the far end speaker stops talking, a break-in circuit in the suppressor enables the send path. Complex problems are associated with echo suppression during periods of double talk, or when one speaker is speaking softly in comparison to the other.
Echo suppression techniques appeared to present an acceptable solution to the problem for local, primarily voice signals. However, for ultra-long distance signals, such as those which include satellite transmissions, delays of 600 milliseconds or longer are encountered and circuits using conventional echo suppressors are significantly worse than comparable circuits with pure delay and no source of echo. The circuits exhibit cutting, fading and other signal changes which result in speech degradation. These effects are attributable to the voice switch operation of echo suppression methods. Representative echo suppressors containing a more in depth explanation of their operation may be found in U.S. Pat. No. 3,937,907 to Campanella.
In response to the known limitations inherent in echo suppression (signal degredation), alternative methods of eliminating this undesirable signal component in the transmitted messages were developed. This new class of devices known as echo cancellers does not interrupt the send line, but generates an approximation y(t), of the echo y(t). The echo approximation y(t) is then subtracted from the actual echo value y(t), and under optimal conditions the difference is zero, resulting in complete echo cancellation. However, both design and practical limitations prohibit complete echo cancellation at all times, as a result the signal appearing on the send line is x(t)+e(t), where x(t) is the local information signal, and e(t) is the residual echo caused by y(t) not being exactly equal to y(t).
An excellent in-depth exposition of the principles of echo cancellation is contained in the paper "Echo Cancellation on Time-Variant Circuits" by N. Demytko and K. English, Proceedings of the IEEE, Vol. 65, No. 3, March 1977, pages 444-453. Briefly summarizing, echo reduction is achieved by applying the receive-in signal to an estimate of the echo path transfer function and subtracting the synthesized replica of the echo from the send-in signal path. Conceptually, the echo component is cancelled without blocking the complete send-side signal.
Generally speaking, echo cancellers are best implemented in digital adaptive filters. The adaptive filter continuously adapts to the echo path impulse response. Real-time algorithms have been developed for determining the optimum coefficients of the adaptive filter. The mathematical basis of the established algorithms is the well-known "method of steepest descent." Increments are added to each coefficient setting in the direction of the negative gradient of an optimality criterion.
Echo cancellers based on the method of steepest descent or gradient search algorithm have been fabricated using VLSI technology. These cancellers inherently include some rather severe limitations, such as slow convergence rate, and poor approximation of the echo signal to be cancelled. Furthermore, the echo cancellation systems previously mentioned were of a high filter order and therefore, require a large fabrication area on VLSI chips. This results in low chip yield and concomitant high cost per unit.
The echo path may be modeled as the system's transfer function. This may be done be generating the system's impulse response or by locating its poles and zeros. The impulse response is a non-parametric n-point discrete approximation of the echo path. A deficiency common to impulse response based cancellers is that the unique characteristics of speech make it difficult to design a canceller with rapid convergence. In addition, these same speech characteristics require that the number of samples needed to reach an acceptable level of echo attenuation is on the order of hundreds, requiring equivalently large numbers of memory locations resulting in large integrated circuit size.
Parametric modeling on the other hand locates the poles and zeros of the echo path, thereby producing an exact model, rather than an approximation as in impulse response-based cancellers. Parametric, or systems identification filter modeling has been suggested, but the saving in number of coefficients required was thought to be offset by the necessarily complex control strategy, and a problem of stability. Demytko, supra at 446, and R. Wehrmann and W. Koch, "Transmission Characteristics of Echo Paths", Nachr. Tech., Vol. 25, page 162, 1972.
A recursive-like adaptive echo canceller has been proposed in U.S. Pat. No. 4,057,696 to Gitlin. Gitlin explicitly recognizes the difficulty of a true recursive adaptive echo canceller arrangement. "An inherent difficulty with the recursive arrangement is that its operation cannot be readily adapted by automatic control in order to minimize the mean squared residual echo. In a practical application, the recursive circuit is predicted not likely to converge to the operating point that will provide the most effective echo cancellation because characteristically there are several sub-optimum multiplier tap settings to which the adaptive algorithm can converge rather than an optimum unique minimum as in the case for the conventional feed forward echo canceller."